1. Field of the Invention
This invention relates to baths for the electroplating of palladium alloys, and in particular to the deposition of iron and cobalt alloys of palladium in a mixed ligand system and a process therefore.
2. Description of Related Art
Electroplating is a well known process for providing a coating upon a substrate to enhance its appearance; to improve its resistance to corrosion, abrasion, or diffusion; or to improve and maintain its solderability. In the electronics industry, precious metals have been used as contact materials to ensure low resistance, noise free contacts which are reliable over time even under severe environmental conditions which would degrade metals such as copper. Due to their cost, precious metals are used only where their desirable properties of corrosion resistance or bondability are needed, and electroplating has proven to be an efficient process to provide a controlled amount of a deposit onto a specified area.
There are always pressures to reduce cost, so palladium is often substituted for gold plating. For connectors and contacts, a controlled degree of hardness and wear resistance is also needed to ensure reliable operation over many cycles. The base metal is typically copper or beryllium copper alloy, which is first plated with 30 to 100 microinches of nickel to limit pinholes and corrosion. Palladium has been plated over nickel to provide a low contact resistance, but palladium alone is relatively soft in applications where many cycles of connector operation are anticipated. A palladium-nickel alloy increases the hardness of the deposit and improves wearability. However, quality control measures often use x-ray fluorescence to determine the thickness and composition of electroplates. So where nickel is present as an underlayer and as a constituent in the electroplate, quality control is made more difficult. These measurements are important because they can determine the corrosion resistance and contact reliability of the plated part. Nickel also has allergenic properties which affect some persons, so the use of palladium-nickel alloys may be restricted in some decorative applications. Consequently, there are applications in which it is more desirable to plate palladium-iron or palladium-cobalt to increase the hardness of the deposit, improve the quality control of the deposit and reduce its allergenic effects.
For example, U.S. Pat. No. 4,242,180 given to Heppner et al. discloses a process and a series of plating baths which deposit palladium and palladium alloys. The palladium is present as a diglycinate palladium II complex, using aminoacetic acid as the sole complexing agent. The bath can also contain conducting salts, buffer reagents, and complexing agents such as ethylenediamine-tetraactetic acid or nitrilotriacetic acid for complexing non-palladium metallic impurities, surface active substances, or the like. Baths with dissolved alloying agents such as nickel, silver, and cobalt are given by way of example.
Ammonia-containing plating baths have been described in the literature by Vinogradov et al. in Zashchita Metallov, Vol. 4, No. 5, pp. 543-547, 1968, and Zashchita Metallov, Vol. 7, No. 5, pp. 612-613, 1971. In the former work, palladium-cobalt alloys were deposited in an amino-chloride electrolyte containing palladium in the form of [Pd(NH3)4]Cl2. Ammonium chloride and sodium hydroxide were added to adjust the pH to a value of 10. Cobalt was added in the form of [Co(NH3)6]Cl2. The wear resistance and internal stress of the Pd--Co alloy increased sharply with the increase of the concentration of cobalt in the bath. In the latter work, cobalt in the form of a pyrophosphate complex derived from K6Co(P2O7)02 was used to lower the internal stress in the alloy of the former work. The pyrophosphate electrolyte more efficiently wets the cathode surface and increases the current yield of the alloy. The deposits became dull and more crystalline above a current density of 1 A/dm2. The recommended bath also includes potassium pyrophosphate, ammonium chloride, and ammonium citrate. These baths were used to study the mechanical properties of the deposits for various plating conditions but they would be difficult to maintain in a production environment because they operate in a narrow range of pH and low current density.
Accordingly, there is a need in the art for a palladium alloy plating bath and process which provides bright, adherent, and ductile deposits which are hydrogen-free and are both chemically and electrochemically stable. The bath should also be versatile enough to deposit a range of alloy compositions ranging from 10% to 95% palladium and be amenable to both high speed plating which is encountered in reel-to-reel plating operations which are typical of plating electrical contacts and slow speed plating operations which are exemplified in rack or barrel plating operations for decorative articles. Additionally, it is desirable to keep the present contact/connector technology which may employ a nickel barrier over a base metal yet not interfere with x-ray fluorescence quality control measures.